Magnesium deficiency in experimental animals produces an anemia and a shortened erythrocyte survival. Erythrocytes from rats deficient in magnesium were examined using freeze-fracture electron microscopy. The results show an erythrocyte membrane abnormality in the form of a plaque which develops after the animals have been on a magnesium-deficient diet for two weeks. These plaques enlarge in size with progressing magnesium deficiency. The plaques are unaltered by in vitro incubation of the red cells with normal concentrations of magnesium.